This invention relates to a rear fork of a motorcycle, and more particularly to a rear fork of the type that a rear end portion of a pair of rear fork arms is formed in a bifurcate shape for supporting a rear axle.
Generally, a rear wheel of a motorcycle is supported by a rear end portion of a rear fork oscillatably attached to a vehicle body frame. The rear fork has a pair of right and left rear fork arms. A rear end portion of each of the rear fork arms is formed in a bifurcate shape in order to easily attach and detach the rear wheel. The rear axle is disposed in this bifurcated portion and supported.
A drive chain tension adjusting plate is pivotably supported on the rear axle. This adjusting plate is a cam plate having a spiral-shaped profile (cam face). On the other hand, a pin is projected from a front location of the bifurcated portion of the rear fork arms. The cam face of the adjusting plate engages the pin. Accordingly, when the adjusting plate is pivoted around the rear axle, depending on the pivoting directions, the rear axle is moved rearward or forward along the bifurcated portion of the rear fork arms. Such rearward or forward movement of the rear axle causes a drive chain stretched between an output shaft of an engine and the rear wheel to get tensioned and loosened. Accordingly, by setting the adjusting plate in an appropriate angular position, tension degree of the drive chain can be properly adjusted.
However, the above-mentioned conventional rear fork includes several problems when the rear fork is going to be made of light alloy in order to reduce its weight. That is, since the bifurcated portion serves to transmit weight of the vehicle body to the rear axle and to transmit the reaction force received from the ground to the vehicle body through the rear axle, the bifurcated portion requires a sufficient strength. If this portion is to be made of light alloy, it is necessarily required to make the wall of this portion thick. As a result, the intended light weight effect to be obtained by using the light alloy material is offset or diminished. In addition, the rear fork is required to be made large in its size, and molding thereof becomes difficult. The pin is required to be made of highly strong materials such as steel in order to bear the tensile force of the chain. However, these materials are difficult to weld to a light alloy material such as aluminum.
In order to detach the rear wheel from the rear fork in order to repair a flat tire, etc., the rear axle is required to be pulled off from the rear wheel and the rear fork arms. At this time, the drive chain tension adjusting plate loses its support and drops. Accordingly, after the rear wheel and the adjusting plate are reattached to the rear fork, the adjusting plate is required to be readjusted. This is particularly disadvantageous when a prompt work is required as in the case of an off-road endurance race, or the like.